An accretion disk is a structure formed by diffused material in orbital motion around a central object, such as a star, black hole, or neutron star. The material—composed of gas, dust, and sometimes other celestial debris—spirals inward toward the central object due to gravitational attraction.
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An accretion disk is an orbiting flow in which stresses permit matter to move inward by transporting angular momentum to other matter or an outflow. Simply losing heat is insufficient for gas that retains too much angular momentum to reach the center. The stresses also dissipate orbital energy, potentially producing strong radiation. A spreading ring illustrates the exchange: some material can accrete while a smaller fraction moves outward carrying the required angular momentum.[ref]